Cored motor

ABSTRACT

A cored motor is composed of a rotor ( 21 ), which is composed of a rotor hub ( 2 ), a shaft ( 1 ) fixed to the rotor hub ( 2 ), a rotor yoke ( 5 ) in ring shape and a ring magnet ( 6 ) fixed to an inner circumferential surface of the rotor yoke ( 5 ), and a stator ( 3 ), which is composed of a motor base ( 13 ) and a stator core ( 14 ) and a sleeve ( 9 ) in cylindrical shape fixed to the motor base ( 13 ) respectively. A surface of the rotor hub ( 2 ) in the motor base side is provided with a recessed portion ( 23 ) in annular shape and a raised portion ( 24 ) that is protruded continuously or intermittently in annular shape. The rotor yoke ( 5 ) is fixed to the recessed portion ( 23 ) and the ring magnet ( 6 ) is contacted with the raised portion ( 24 ) and fixed to the rotor yoke ( 5 ). Respective center lines of the rotor yoke ( 5 ), the ring magnet ( 6 ) and the stator core ( 14 ) in a longitudinal direction along the shaft ( 1 ) are made approximately coincide with each other.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a cored motor, particularly, relates toa cored motor having a dynamic pressure bearing, which is suitable fordriving a disc installed in a hard disc drive (HDD).

2. Description of the Related Art

A typical example of a conventional cored motor is disclosed in theJapanese publication of unexamined patent applications No. 10-4642/1998.The cored motor disclosed in the publication is composed of a shaft, astator case, a rotor core that is installed with laminated cores, and amagnet that is mounted on the inner surface of the stator case so as toconfront with the outer circumferential surface of the rotor core.

Further, there existed another example of a cored motor for HDDaccording to the prior art shown in FIGS. 4 and 5.

With referring to FIGS. 4 and 5, the cored motor for HDD according tothe prior art is explained.

FIG. 4 is a cross-sectional view of a cored motor according to the priorart.

FIG. 5 is a fragmentary cross-sectional view, with enlarging a part ofthe cored motor indicated by a circle “B” in FIG. 4.

In FIGS. 4 and 5, a cored motor is composed of a stator 103 and a rotor121 that is provided with a rotor hub 102, which is installed with adisc 26.

The stator 103 is further composed of a motor base 113, a stator core114 that is fixed to the motor base 113, and a sleeve 109.

The sleeve 109 sustains a shaft 101 through a dynamic pressure bearingso as to rotate freely as well as being fixed to a center hole 113 a ofthe motor base 113, wherein description of the dynamic pressure bearingis omitted.

On the other hand, the rotor 121 is further composed of the rotor hub102, an annular rotor yoke 105, and a ring magnet 106. These rotor 121,annular rotor yoke 105 and ring magnet 106 are assembled such that theannular rotor yoke 105 is fixed to an outer circumferential area of therotor hub 102 on the bottom and the ring magnet 106 that iselectro-painted is affixed on the inner circular side surface of theannular rotor yoke 105. In this assembling process, as shown in FIG. 5,positioning of the ring magnet 106 in an axial direction along the shaft101 is limited by a stopper section 117, wherein the stopper section 117is provided on an inner circumferential area of the annular rotor yoke105.

Further, the shaft 101 is press-fitted into the rotor hub 102 and therotor 121 enables to rotate freely with respect to the stator 103.

A section of the annular rotor yoke 105 is formed in an inverted “L”shape having a crooked section 112 in the rotor hub 102 side, whereinthe crooked section 112 extends toward a center of the annular rotoryoke 105. The crooked section 112 is formed by the drawing process.

Further, the crooked section 112 effectively protects a recording andreproducing head 150 of an HDD from being affected by magnetic fluxpossibly leaking from the ring magnet 106.

Furthermore, a clearance section 108 is provided between the ring magnet106 and the annular rotor yoke 105 so as to reduce magneticshort-circuiting by reducing a contact area between them. The clearancesection 108 is also provided for relieving a caulking jig when acaulking process is applied to the rotor hub 102.

With referring to FIG. 5, a positional relationship in the axialdirection among the rotor yoke 105, the ring magnet 106, and the statorcore 114 is detailed next.

In FIG. 5, the ring magnet 106 and the stator core 114 is allocated suchthat a center line 118 of the ring magnet 106 having a length L102 inthe longitudinal direction along the shaft 101 is shifted upward by adistance DL with respect to another center line 119 of the stator core114 having a thickness L103 in the longitudinal direction along theshaft 101. Shifting the center line 118 of the ring magnet 106 upward iscaused by canceling shift of a magnetic center of the rotor 121 side. Ifthe center lines 118 and 119 are coincided with each other, by thecrooked section 112 of the rotor yoke 105, a magnetic center of therotor 121 side is shifted downward with respect to a magnetic center inthe longitudinal direction along the shaft 101 of the stator core 114.

In the meanwhile, as mentioned above, the conventional cored motor wasprovided with the crooked section 112 on the rotor yoke 105 in order toprevent the recording and reproducing head 150 from being affected bymagnetic flux possibly leaking from the ring magnet 106.

Further, magnetic flux density of the ring magnet 106 was diminished bythe stopper section 117, which short-circuited a part of magneticcircuit in the rotor hub 102 side. Therefore, as mentioned above, theclearance section 108 was provided so as to relieve a certain level ofdiminishing magnetic flux density.

However, it was necessary for a forming process of the rotor yoke 105 toprovide an extra process for forming the crooked section 112 and theclearance section 108, and resulting in increasing cost of componentparts.

Further, since the crooked section 112 was formed by the drawingprocess, there existed another problem in the manufacturing process suchthat forming the crooked section 112, the stopper section 117, and theclearance section 108 in higher dimensional accuracy was extremelydifficult.

In the mass production of the conventional cored motor, dimensionalaccuracy of positioning the ring magnet 106 and the rotor yoke 105fluctuated extremely. In this connection, magnetic flux density wasdeviated to the upper side of the ring magnet 106, toward the rotor yoke105, or to the lower side of the ring magnet 106, toward the motor base113. Consequently, it was difficult to coincide the magnetic center ofthe ring magnet 106 with the magnetic center of the stator core 114accurately and without any deviation.

In this case, there existed a further problem such that the rotor hub102 has rotated with being sucked in either axial direction of upward ordownward, and resulting in making vibration and noise larger due tounbalanced rotation of the rotor hub 102.

Further, in the case that a posture of an apparatus installing theconventional cored motor is indefinite, balanced revolution of a rotorhub of a cored motor is intentionally unbalanced by making the rotor hubto be sucked in the axial direction so as to reduce vibration and so asnot to degrade revolving efficiency in spite of the posture of theapparatus. Such a cored motor was disclosed in the Japanese publicationof unexamined patent applications No. 10-4642/1998.

However, particularly, in the case that such a cored motor is installedin a HDD, actually, posture of the HDD is almost fixed. Consequently, itis most important for the conventional cored motor to balance therevolution of the rotor hub of the cored motor by coinciding themagnetic center of the ring magnet with the magnetic center of thestator core.

Further, in the case that a bearing of the cored motor is a dynamicpressure bearing, there exists a furthermore problem in connection withreliability such that unbalanced force in the axial direction, whichoccurs when the balanced revolution of the rotor hub collapses, resultsin shortening life of the dynamic pressure bearing extremely.

SUMMARY OF THE INVENTION

Accordingly, in consideration of the above-mentioned problems of theprior art, an object of the present invention is to provide a coredmotor, which is low in vibration and noise, long in life of a dynamicpressure bearing, easy to manufacturing, and low in cost.

In order to achieve the above object, the present invention provides,according to an aspect thereof, a cored motor comprising a rotor beingcomposed of a rotor hub, a shaft fixed to the rotor hub, a rotor yoke inring shape and a ring magnet fixed to an inner circumferential surfaceof the rotor yoke; and a stator being composed of a motor base, and astator core and a sleeve in cylindrical shape fixed to the motor baserespectively, wherein the shaft is inserted into the sleeve so as tosupport the rotor to be rotatable freely with respect to the statorwhile the ring magnet is kept confronting with the stator core, andwherein a surface of the rotor hub in the motor base side is providedwith a recessed portion in annular shape and a raised portion protrudedcontinuously or intermittently in annular shape, and wherein the rotoryoke is fixed to the recessed portion, and wherein the ring magnet iscontacted with the raised portion and fixed to the rotor yoke, and thecored motor is characterized in that respective center lines of therotor yoke, the ring magnet and the stator core in a longitudinaldirection along the shaft are made approximately coincide with eachother.

According to another aspect of the present invention, there provides acored motor, which comprises a rotor being composed of a rotor hub, asleeve in cylindrical shape fixed to the rotor hub, a rotor yoke in ringshape and a ring magnet fixed to an inner circumferential surface of therotor yoke; and a stator being composed of a motor base, and a shaft anda stator core fixed to the motor base respectively, wherein the shaft isinserted into the sleeve so as to support the rotor to be rotatablefreely with respect to the stator while the ring magnet is keptconfronting with the stator core, and wherein a surface in the motorbase side of the rotor hub is provided with a recessed portion inannular shape and a raised portion protruded continuously orintermittently in annular shape, and wherein the rotor yoke is fixed tothe recessed portion, and wherein the ring magnet is contacted with theraised portion and fixed to the rotor yoke, and the cored motor ischaracterized in that respective center lines of the rotor yoke, thering magnet and the stator core in a longitudinal direction along theshaft are made approximately coincide with each other.

Other object and further features of the present invention will beapparent from the following detailed description when read inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross sectional view of a cored motor according to anembodiment of the present invention.

FIG. 2 is a fragmentary cross-sectional view, with enlarging a part ofthe cored motor indicated by a circle “A” in FIG. 1.

FIG. 3(a) is a graph exhibiting effect of the cored motor according tothe embodiment shown in FIG. 1 in comparison with a conventional coredmotor according to the prior art as a comparative example.

FIG. 3(b) is a comparison chart between the embodiment and thecomparative example based on the graph shown in FIG. 3(a).

FIG. 4 is a cross-sectional view of a conventional cored motor accordingto the prior art.

FIG. 5 is a fragmentary cross-sectional view, with enlarging a part ofthe conventional cored motor indicated by a circle “B” in FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS EMBODIMENT

FIG. 1 is a cross sectional view of a cored motor according to anembodiment of the present invention.

FIG. 2 is a fragmentary cross-sectional view, with enlarging a part ofthe cored motor indicated by a circle “A” in FIG. 1′.

FIG. 3(a) is a graph exhibiting effect of the cored motor according tothe embodiment shown in FIG. 1 in comparison with a conventional coredmotor according to the prior art as a comparative example.

FIG. 3(b) is a comparison chart between the embodiment and thecomparative example based on the graph shown in FIG. 3(a).

In FIGS. 1 and 2, a cored motor (hereinafter referred to as motor) iscomposed of a rotor 21 having a rotor hub 2 and a stator 3, wherein adisc 26 for magnetic recording and reproducing is mounted on the rotorhub 2 in a later process.

The stator 3 is further composed of a motor base 13, a sleeve 9 that isprovided on the motor base 13 with standing upright, and a stator core14 that is provided with a coil 15.

The rotor 21 is composed of the rotor hub 2 and a shaft 1 that is fixedto the rotor hub 2.

In the stator 3, a cylindrical section 13 a is formed on the motor base13 for mounting the stator core 14, and a stepped section 25 is providedin the cylindrical section 13 a.

The stator core 14 is fixed to the stepped section 25 so as to beextremely excellent in dimensional accuracy.

Further, the stator core 14 is formed by laminating eight sheets ofsilicon steel plate having a thickness of 0.35 mm and a total thicknessL3 of the stator core 14 is 2.8 mm.

Furthermore, the coil 15 is wound around the stator core 14.

The sleeve 9 is fixed to a center hole 13 b of the cylindrical section13 a. The sleeve 9 sustains the shaft 1 as a bearing, so that the rotor21 enables to rotate around the stator 3 freely.

The shaft 1 is made from a material of martensite based, ferrite based,or austenite based stainless steel.

Further, the shaft 1 is applied with surface coating such as electrolessnickel plating. A thickness of the surface coating applied to the shaft1 is the order of 3 to 50 μm.

In the rotor 21, a rotor yoke 5 in ring shape is securely affixed on abottom surface of the rotor hub 2.

Further, a ring magnet 6 is securely affixed on an inner circumferentialsurface of the rotor yoke 5 so as to confront with the stator core 14with maintaining a prescribed gap between the ring magnet 6 and thestator core 14. Details of each shape of the rotor hub 2, the rotor yoke5 and the ring magnet 6, and dimensional relationship between them willbe explained later.

In the motor according to the embodiment of the present invention, theshaft 1 is sustained rotatably by radial and thrust dynamic pressurebearings. With referring to FIG. 1, a dynamic pressure bearing isdetailed next.

In FIG. 1, radial dynamic pressure bearings 22 and 23 are constituted bythe shaft 1 and the sleeve 9.

In case of the embodiment shown in FIG. 1, an inner circumferentialsurface of the sleeve 9 is formed with a dynamic pressure groove such asherringbone and Rayleigh step, which is provided for generating dynamicpressure in the radial direction, as shown by herringbone patterns inFIG. 1.

Further, it should be understood that the dynamic pressure grooveenables to be formed on the outer circumferential surface of the shaft1.

A prescribed gap is provided between the outer circumferential surfaceof the shaft 7 and the inner circumferential surface of the sleeve 9 andfilled with lubricant. By intervening of the lubricant, the dynamicpressure in the radial direction is generated in accordance withrevolution of the rotor 21, that is, revolution of the shaft 1.Consequently, the rotor hub 2 in revolution is sustained in the radialdirection.

On the other hand, an axial dynamic pressure bearing is constituted by aflange 7 that is fixed to a bottom end portion of the shaft 1 and aplate 10 that is affixed on a bottom end portion of the sleeve 9 so asto hermetically shield between the bottom end portion of the sleeve 9and the flange 7.

Further, an axial dynamic pressure groove (not shown) is formed on topand bottom surfaces of the flange 7, a bottom end surface of the sleeve8 confronting with the flange 7, and a top surface of the plate 10. Theaxial dynamic pressure groove is such a dynamic pressure groove asherringbone and Rayleigh step and formed by the processing method suchas etching, stamping and pressing.

Furthermore, a gap is provided between the upper surface of the flange 7and the bottom end surface of the sleeve 9 confronting with the flange 7and between the bottom surface of the flange 7 and the top surface ofthe plate 10 respectively, and then lubricant is filled in these gaps.By intervening of the lubricant, the dynamic pressure in the axialdirection is generated in accordance with revolution of the rotor 21,that is, revolution of the shaft 1. Consequently, the rotor hub 2 inrevolution is sustained in the axial direction.

Succeedingly, details are given to the stator 3.

As mentioned above, the stator 3 is composed of the motor base 13, thestator core 14 that is fixed to the motor base 13 and the coil 15 thatis wound around the stator cored 14. The motor base 13 is formed by thealuminum diecasting method, and then a prescribed portion is shaved off.The stator core 14 is applied with insulative coating through aprocessing method such as electro-painting and powder coating.

A terminal 15 a of the coil 15 is soldered on a flexible printed circuitboard (hereinafter referred to as FPC) 16 through a through hole 11provided on the motor base 13. The FPC 16 is provided with a portion atwhich the terminal 15 a is soldered and a land portion for connecting adriving circuit (not shown) of a hard disc drive (HDD) through aconnector. These portions are electrically connected by way of a printedcircuit pattern on the FPC 16.

In the above-mentioned configuration, by applying electricity to thedriving circuit (not shown) of the HDD, electricity is sequentiallysupplied to each phase winding of the coil 15, and resulting in rotatingthe rotor 21.

Higher accuracy and reliability are demanded particularly for a motor tobe used in a HDD for driving to rotate a disc. In accordance withincreasing in recording capacity of a disc, the motor equipped with thedynamic pressure bearing according to the embodiment of the presentinvention is suitable for such a motor to be used in a HDD because thedynamic pressure bearing is excellent in an NRRO (Non Repeatable RunOut) characteristic in comparison with the conventional ball bearing.

With respect to the rotor yoke 5, the ring magnet 6 and the stator core14 as an essential part, details are given to them next.

A recessed portion 28 in annular shape is provided on a bottom surfaceof the rotor hub 2 that is made from a non magnetic aluminum basedmaterial, wherein the recessed portion 28 is concaved upward in acircular recess. An upper end portion of the rotor yoke 5 is fit intothe recessed portion 28.

The rotor yoke 5 is formed in an annular shape having a uniformthickness. A width L1 in the longitudinal direction of the rotor yoke 5is 5 mm. The rotor yoke 5 is made from a magnetic iron based materialand applied with nickel plating on the surface.

Further, a raised portion 24 in annular shape, which is protrudeddownward, is provided inside the recessed portion 28 that is formed onthe bottom surface of the rotor hub 2. A top end portion of the ringmagnet 6 is contacted with the raised portion 24, and resulting inrestricting a disposition of the ring magnet 6 in the axial direction ofthe shaft 1. It should be understood that the raised portion 24 is notlimited to a ring shape. A shape protruding intermittently in the radialdirection is also acceptable for the raised portion 24. In other words,any shape is acceptable for the raised portion 24 as far as the shapeenables to be stably contacted with the ring magnet 6.

The ring magnet 6 is formed in the annular shape so as to be fit insidethe rotor yoke 5 without rattling. A width L2 of the ring magnet 6 inthe axial direction of the shaft 1 is 4 mm. The ring magnet 6 is madefrom a Nd—Fe—B (neodymium-iron-boron) based material, andelectro-painted on the surface.

It should be understood that the recessed portion 28 and the raisedportion 24 enables to be formed by transferring a molding die throughthe aluminum diecasting process or by shaving process as a secondaryprocessing.

The rotor yoke 5 is fit into the recessed portion 28 and affixedtherein. On the other hand, the ring magnet 6 is affixed on the innerwall surface of the rotor yoke 5 so as to be securely fastened thereonwhile the ring magnet 6 is contacted with the raised portion 24.Consequently, the rotor hub 2, the rotor yoke 5 and the ring magnet 6 isassembled in one unit.

A chamfered portion 27, which is chamfered by 0.5 mm, for example, isprovided at each corner of both the top and bottom end portions in theinner circumferential area of the rotor yoke 5, so that the ring magnet6 enables to be easily inserted into or fit into the rotor yoke 5.

With respect to the above-mentioned process of assembling in one unit,each dimension of the rotor yoke 5, the ring magnet 6 and the statorcore 14 is designated such that each of a center line 18 of the rotoryoke 5 having the width L1 in the longitudinal direction along the shaft1, a center line 19 of the ring magnet 6 having the width L2 in thelongitudinal direction along the shaft 1 and a center line 20 of thestator core 14 having the total thickness L3 in the longitudinaldirection along the shaft 1 coincides with each other.

By this designation, the rotor yoke 5, the ring magnet 6 and the statorcore 14, that is, all magnetic substances related to driving the motorare symmetrical with respect to a plane that includes the center line 18of the rotor yoke 5 having the width L1 in the longitudinal directionalong the shaft 1.

Consequently, a magnetic circuit also becomes symmetrical with respectto the plane, and resulting in reducing vibration and noise extremelywhile the rotor 21 is rotating.

Further, since the width L1 in the longitudinal direction of the rotoryoke 5 is longer than the width L2 of the ring magnet 6, the top andbottom end portions of the rotor yoke 5 protrude from those of the ringmagnet 6 respectively in case the center line 18 of the rotor yoke 5 andthe center line 19 of the ring magnet 6 is disposed so as to coincidewith each other A recording and reproducing head 150 of the HDD that isinstalled with the motor is allocated obliquely above the ring magnet 6,so that magnetic flux possibly leaking from the ring magnet 6 iseffectively shielded by the top end portion of the rotor yoke 5, whichprotrudes upward from the top end portion of the ring magnet 6.Consequently, the recording and reproducing head 150 is prevented frommagnetic affection caused by leaked magnetic flux from the ring magnet6.

Further, the rotor yoke 5 is formed in a ring shape without a crookedsection, so that the rotor yoke 5 enables to be formed by a simplemanufacturing method such as pressing a plane sheet, cutting a tubularmaterial and rolling up a plane sheet in strips. Consequently, the rotoryoke 5 enables to be manufactured less in manufacturing processes andlow in manufacturing cost. Particularly, the manufacturing method ofcutting a tubular material is the most desirable method because an innerdiameter of the tubular material enables to be maintained in higheraccuracy, and resulting in fitting the ring magnet 6 into the rotor yoke5 excellently.

In mass production, it possibly occurs that dimensional accuracy of eachcomponent part varies. However, in the case of the motor according tothe embodiment of the present invention, a magnetic circuit enables tobe defined as perfectly symmetric although the center lines 18 and 19 ofthe rotor yoke 5 and the stator core 14 are deviated within ±0.196 mmfrom the center line 20 of the ring magnet 6 on the basis of aparticular case that respective center lines 18 and 19 of the rotor yoke5 and the stator core 14 in the longitudinal direction along the shaft 1perfectly coincide with respect to the center line 20 of the ring magnet6, wherein the amount±0.196 mm is equivalent to ±7% of the total widthL3, that is, 2.8 mm of the stator core 14. This is caused by that themotor of the present invention is excellently low in vibration and noisewithin the deviation range of ±0.196 mm. Consequently, it issubstantially recognized that the motor is excellently balanced.

Further, with respect to the chamfered portion 27 of the rotor yoke 5,the specific case of providing the chamfered portion 27 on the top andbottom end portions of the rotor yoke 5 is illustrated in FIG. 2.However, a chamfered shape formed on the end portion of the rotor yoke 5hardly affects a magnetic circuit, so that symmetry of the magneticcircuit is never damaged although the chamfered portion 27 is providedonly on the bottom end portion of the rotor yoke 5 confronting with themotor base 13.

20 each of the motors according to the present invention and theconventional cored motors according to the prior art as a comparativeexample are experimentally manufactured, and vibration of the motors aremeasured by using a vibrometer when they are driven by individual threerotational frequencies (rpm). Results of the measurement are shown inFIGS. 3(a) and 3(b). As shown in FIGS. 3(a) and 3(b), the motoraccording to the embodiment of the present invention is excellent inmean value and standard deviation in comparison with the comparativeexample. In other words, with respect to the motor according to theembodiment of the present invention, scattering of vibration at anyrotational frequencies is superior to that of the comparative example.

It is apparent that the motor according to the present invention issuperior to the conventional cored motor according to the prior art invibration. Particularly, a maximum value of vibration that is the mostimportant factor for evaluating a motor characteristic is drasticallyreduced in the motor according to the present invention.

Accordingly, the motor of the present invention is drastically improvedin vibration and small in scattering of vibration during massproduction.

As mentioned above, according to the present invention, there provided acored motor, which is small in vibration and noise.

Further, the cored motor is provided with a dynamic pressure bearing,which is long in life and results in higher reliability.

Furthermore, the cored motor is easy to manufacture and low inmanufacturing cost.

While the invention has been described above with reference to aspecific embodiment thereof, it is apparent that many changes,modification and variations in materials and the arrangement ofequipment and devices can be made without departing from the inventionconcept disclosed herein. For example, the embodiment is described onlyin a cored motor suitable for driving a disc installed in a hard discdrive (HDD). However, the cored motor according to the present inventionis applied for not only an HDD but also driving an optical disc and apolygon mirror as a motor in general.

Further, the embodiment of the present invention is described in a motorin which the shaft 1 is fixed to the rotor hub 2, that is, a so-calledshaft rotational type motor. However, it is also acceptable for a motorin which the shaft 1 is fixed to the motor base 13, that is, a so-calledshaft fixed motor.

It should be understood that many modifications and adaptations of theinvention will become apparent to those skilled in the art and it isintended to encompass such obvious modifications and changes in thescope of the claims appended hereto.

1. A cored motor comprising: a rotor being composed of a rotor hub, ashaft fixed to the rotor hub, a rotor yoke in ring shape and a ringmagnet fixed to an inner circumferential surface of the rotor yoke; anda stator being composed of a motor base, and a stator core and a sleevein cylindrical shape fixed to the motor base respectively, wherein theshaft is inserted into the sleeve so as to support the rotor to berotatable freely with respect to the stator while the ring magnet iskept confronting with the stator core, and wherein a surface of therotor hub in the motor base side is provided with a recessed portion inannular shape and a raised portion protruded continuously orintermittently in annular shape, and wherein the rotor yoke is fixed tothe recessed portion, and wherein the ring magnet is contacted with theraised portion and fixed to the rotor yoke, and the cored motor ischaracterized in that respective center lines of the rotor yoke, thering magnet and the stator core in a longitudinal direction along theshaft are made approximately coincide with each other.
 2. A cored motorcomprising: a rotor being composed of a rotor hub, a sleeve incylindrical shape fixed to the rotor hub, a rotor yoke in ring shape anda ring magnet fixed to an inner circumferential surface of the rotoryoke; and a stator being composed of a motor base, and a shaft and astator core fixed to the motor base respectively, wherein the shaft isinserted into the sleeve so as to support the rotor to be rotatablefreely with respect to the stator while the ring magnet is keptconfronting with the stator core, and wherein a surface in the motorbase side of the rotor hub is provided with a recessed portion inannular shape and a raised portion protruded continuously orintermittently in annular shape, and wherein the rotor yoke is fixed tothe recessed portion, and wherein the ring magnet is contacted with theraised portion and fixed to the rotor yoke, and the cored motor ischaracterized in that respective center lines of the rotor yoke, thering magnet and the stator core in a longitudinal direction along theshaft are made approximately coincide with each other.